More than half of the world's major forests will be lost if global temperatures rise by an average of 3C or more by the end of the century, it was claimed yesterday. The prediction comes from the most comprehensive analysis yet of the potential effects of human-made global warming.

Extreme floods, forest fires and droughts will also become more common over the next 200 years as global temperatures rise owing to climate change, according to Marko Scholze of Bristol University. Dr Scholze took 52 simulations of the world's climate over the next century, based on 16 different climate models, grouping the results according to varying amounts of global warming they predicted by 2100: less than 2C on average, 2C-3C and more than 3C. He then used the simulations to work out how the world's plants would be affected over the next few hundred years. The results were published yesterday in the Proceedings of the National Academy of Sciences.

Alan O'Neill, science director for the National Centre for Earth Observation, said: "Some work in this area has been done before looking at the meteorological forecasts for climate change and feeding those into vegetation models ... this is a much more comprehensive study."

He added that Dr Scholze's results would give climate scientists the most accurate scientific projection yet of the future effects of global warming.

Dr Scholze said the effects of a 2C category were inevitable. This is the temperature rise that will happen, on average, even if the world immediately stopped emitting greenhouse gases. This scenario predicts that Europe, Asia, Canada, central America and Amazonia could lose up to 30% of its forests.

A rise of 2C-3C will mean less fresh water available in parts of west Africa, central America, southern Europe and the eastern US, raising the probability of drought in these areas. In contrast, the tropical parts of Africa and South America will be at greater risk of flooding as trees are lost. Dr Scholze says a global temperature rise of more than 3C will mean even less fresh water. Loss of forest in Amazonia and Europe, Asia, Canada and central America could reach 60%.

A 3C warming could also present a yet more dangerous scenario where the temperatures induce plants to become net producers of carbon dioxide. "As temperatures go up, plants like it better and they start to grow more vigorously and start to take up more carbon dioxide from the air," Dr O'Neill said. "But there comes a point where the take-up is saturated for a given vegetation cover, then the ecosystem starts to respire more than it's taking up."

Dr Scholze's work shows that this so-called "tipping point" could arrive by the middle of this century. His scenarios echo research from the UK's Hadley Centre, a world leader in climate change modelling. In a report published last year called Avoiding Dangerous Climate Change, scientists at the centre predicted that a 3C rise in average temperatures would cause a worldwide drop in cereal crops of between 20m and 400m tonnes, put 400 million more people at risk of hunger, and put up to 3 billion people at risk of flooding and without access to fresh water supplies.

In May, David King, the government's chief scientific adviser, warned that the world's temperature would rise by 3C, causing catastrophic damage around the world, unless governments took urgent action to reduce carbon emissions.

Dr Scholze said his work could help to define the concept of dangerous climate change for policymakers. "Dangerous is very objective. We tried to define a dangerous level and see what the risks are," he said. In his definition, climate change becomes dangerous when an event - such as extreme flooding or heatwaves - that only happened once every 100 years becomes one that happens every 10 years.

He added that a rise of 3C was not inevitable. "We can't just do what we do at the moment, what we call business as usual. We have a few decades - we have to do something before 2040."

Burning issue

At the rate we are burning fossil fuels, global temperatures could easily increase by more than the 3C rise that Marko Scholze's research warns could increase flooding, forest fires and droughts. A 2001 report by the Intergovernmental Panel on Climate Change said an increase of between 1.4 and 5.8C by 2100 would be caused if current carbon emissions continue.

Global sea levels would rise by between 0.09 and 0.88 metres as a result. Scientists at the UK Climate Impacts Programme predict that a 3C rise or above would reduce rain on the south coast to half of current levels, by more than 40% across the rest of England and 30% in Scotland.

Sea levels could be 70cm higher in the south and there would be a 17-fold increase in flooding on the east coast. London could face a £25bn clean-up bill after a storm surge that would overwhelm the Thames barrier.